1. Field of the Invention
The invention relates to a control apparatus and method for an internal combustion engine, and particularly to a control apparatus and method for an internal combustion engine that includes upstream catalysts provided for multiple cylinder groups, respectively, and a downstream catalyst that is provided in an exhaust passage downstream of the confluence of exhaust passages provided downstream of the upstream catalysts, respectively.
2. Description of the Related Art
For example, Japanese patent application publication No. JP-A-08-144814 describes a control apparatus that performs fuel cut when the internal combustion engine is decelerating. According to this control apparatus, execution of the fuel cut is prohibited when the temperature of the catalyst provided in the exhaust passage is high. During fuel cut, a lean gas is supplied to the catalyst, and degradation of the catalyst is promoted when the catalyst is heated to a high temperature under a lean atmosphere. Accordingly, the control apparatus in this publication can minimize such degradation of the catalyst by prohibiting fuel cut as described above.
For example, an internal combustion engine, typically a V-type engine, employs an arrangement of exhaust components in which upstream catalysts are separately provided for two cylinder groups and a downstream catalyst is provided in an exhaust passage downstream of the confluence of the exhaust passages extending downstream of the respective upstream catalysts. Also in an internal combustion engine having this arrangement, the control method described in the above-mentioned publication may be implemented in order to minimize degradation of the catalysts.
In the this case, however, if control is performed so as to avoid the upstream catalysts being heated to a high temperature under a lean atmosphere as described above, the downstream catalyst is exposed to a rich atmosphere. That is, catalysts absorb sulfur oxides under a lean atmosphere and release the absorbed sulfur oxides under a rich atmosphere, and therefore avoiding the upstream catalysts being heated under a lean atmosphere in the above-described manner may promote production of catalyst exhaust odor.
On the other hand, if control is performed so as to avoid the downstream catalyst being exposed to a rich atmosphere in order to minimize catalyst exhaust odor, it in turn promotes degradation of the upstream catalysts, and eventually degradation of the downstream catalyst as well. That is, in an internal combustion engine having the above-described arrangement of exhaust components, it has been difficult to perform control for minimizing overall catalyst degradation and control for minimizing catalyst exhaust odor in a compatible manner.